The present invention relates to magnetoresistive heads for reading information from a magnetic recording medium.
The following patent documents are referred to below by ordinal number, and are hereby incorporated by reference:                1. Japanese Patent Laid-open Hei 3-125311;        2. U.S. Pat. No. 4,663,685; and        3. Japanese Patent Laid-open Hei 11-273030        
In a hard disk drive (HDD), a magnetic head is used for reading information on a magnetic recording medium (i.e., a hard disk) or for writing information. The magnetic head comprises a write head for writing information as magnetization signals to a magnetic recording medium and a read head for reading signals recorded as magnetization signals in the magnetic recording medium. The read head comprises a magnetoresistive stack having a multiplicity of magnetic thin films and nonmagnetic thin films and is referred to as a magnetoresistive head since the device reads signals by utilizing the magnetoresistive effect. The magnetoresistive head has several types of stacked structures which are classified, for example, as AMR head, GMR head, CPP-GMR head and TMR head in view of the principle of magnetic resistance used therefor. They takeout input magnetic fields entered from a magnetic recording medium to a read head as a change of voltage by using AMR (magnetoresistive effect), GMR (Giant Magnetoresistive effect), CPP-GMR effect (Current Perpendicular Plane GMR effect) and TMR (Tunnel Magnetoresistive effect), respectively.
In the magnetoresistive stack of a magnetoresistive head, a magnetic layer in which the magnetization rotates in response to the input magnetic field from the magnetic recording medium is referred to as a free layer. To suppress various kinds of noise such as Barkhausen noise or to control the asymmetry of output, it is important to make the free layer into a single magnetic domain in the direction of track width. When the free layer is not formed into a single magnetic domain and has magnetic domains, magnetic wall movement occurs in response to the input magnetic fields from the magnetic recording medium, causing noise.
Examples of methods of magnetic domain control for making the free layer into a single magnetic domain include a method, as shown in Patent Document 1 for example, of disposing magnetic domain control films comprising a magnetic film on both ends of the free layer and using the magnetic field in the direction of track width generated from the magnetic films. FIG. 10 shows a schematic view of a magnetoresistive head, as viewed from an air bearing surface, subjected to magnetic domain control by this method. A free layer 2 is disposed by way of a nonmagnetic layer 3 above a soft magnetic film 4 (referred to as a pinned layer) in which magnetization is fixed by an antiferromagnetic film 5 and a cap layer 1 is put on free layer 2 in order to prevent it from oxidation. The width of free layer 2 is referred to as a geometrical track width Twr_geo.
Both ends of the magnetoresistive stack comprising layers 1 to 5 are etched by milling or the like to show a trapezoidal device shape viewed from the air-bearing surface as shown in FIG. 10. The structure has a feature by which magnetic domain control films 8 each comprising a magnetic film are disposed at both ends of the device by an underlying film 9. Electrodes 6 are stacked by means of an underlying layer film 7 above the magnetic domain control films. In this structure, the magnetization distribution in free layer 2 is controlled using a magnetic field generated by magnetic domain control films 8 to make the free layer into a single magnetic domain.
Further, another magnetic domain control method includes, for example, a method of stacking antiferromagnetic films on both ends of a long free layer and using exchange coupling between the antiferromagnetic film and the free layer as disclosed in Patent Document 2. FIG. 11 shows a schematic view of a magnetoresistive head subjected to magnetic domain control as viewed from an air-bearing surface. The structure has a feature where a free layer 2 is disposed by means of a nonmagnetic layer 3 above a soft magnetic film 4 (referred to as a pinned layer) in which magnetization is fixed by an antiferromagnetic film 5, and antiferromagnetic films 12 are stacked on both ends of free layer 2. Magnetic domain control is performed by exchange interaction between antiferromagnetic film 12 and free layer 2. Free layer 2 is made larger than the track width written on a magnetic recording medium and the end region is fixed. Accordingly, in this structure, recording is read by a portion (referred to as a magnetically sensitive portion) Tw of the free layer between the antiferromagnetic films. Electrode films 10 are stacked by means of an underlayer film 11 above antiferromagnetic film 12. Underlying film 11 may be saved.
Since the latter method of using the antiferromagnetic field is extremely difficult in view of the process of stacking the antiferromagnetic films on both ends of the free layer, the former method of using the magnetic films is generally used at present.
FIG. 10 shows the magnetic domain control system in which the magnetic domain control films are disposed on both ends of the free layer. In this system, if the magnetic domain control force is increased to ensure the stability of a device output, the magnetic field intensity at the ends where the magnetic domain control film is in contact with the free layer is excessively strong. Consequently, a region is increased where the magnetization of the free layer tends to be rotated less, relative to the magnetic film medium. That is, the dead zone is increased. The dead zone region can be decreased simply by weakening the magnetic domain control force, for example, by merely decreasing the film thickness. In this case, however, since this results in disadvantages such as generation of Barkhausen noise, generation of waveform fluctuations caused by hysteresis in transfer curves or increase in output asymmetry, appropriate magnetic domain control force is necessary. The transfer curve shows the relation between the magnitude of the input magnetic field and the head output, which expresses basic input/output characteristics of the magnetic head. The output changes must be linearly relative to the input magnetic field. Accordingly, a desirable transfer curve is linear.
When using the magnetic domain control system to dispose magnetic domain control films on both sides of the free layer as shown in FIG. 10, it is necessary to make the gradient of the transfer curve abrupt while keeping the linearity of the transfer curve. For this purpose, it is necessary to optimize the magnetization film thickness product of the control film. For example, as shown in Patent Document 3, there is a method of setting the residual magnetization film thickness product of a permanent magnetic film in accordance with values for the read track width, the read gap film thickness and the saturated magnetization film thickness product of the magnetic sensitizing layer. Definition for the magnetization ratio in Patent Document 3 concerns an MR head having a soft adjacent layer (SAL) film in which a calculation model is prepared based on experimental values using an MR head with a track width of 500 nm or more and the result is put in to obtain a mathematical formula. Accordingly, it is difficult to apply the definition to a narrow track magnetoresistive GMR head.
In recently produced magnetic head layers, the track width is extremely narrow and the entire free layer tends to be easily and uniformly rotated by exchange coupling. In addition, the magnetic domain control magnetic field is applied not only to a portion of the layer in the vicinity of the magnetic domain control film but to the entire free layer. It is therefore necessary to strictly determine the magnetic film thickness product.